(1) Field of the Invention
The present invention relates to a sheet feed control method, and particularly to a sheet feed control method which is excellent in sheet feed accuracy.
(2) Prior Art
Generally, apparatuses, for example, such as a facsimile, a printer, a video printer or the like for outputting various information often are of the type for outputting information to a sheet. Such an output apparatus is provided with a sheet feed mechanism for feeding a sheet which is a medium for outputting information.
In this sheet feed mechanism, there are various types according to uses or the like. In any of them, it is necessary to feed a sheet a predetermined pitch with high accuracy.
Particularly recently, there has been proposed a thermal printer in which a heat generating element is formed to have a length for plural lines in order to increase the effective printing speed thereof, so that printing for plural lines are simultaneously carried out by a single reciprocation of a carriage.
In such a thermal printer as described above, normally, the sizes of characters to be printed, pitches between lines and the like are constant, and therefore, sheet feed is carried out by the same distance as a height of a heat generating element of a thermal head. Accordingly, characters in a line are printed by two reciprocations of the carriage depending on the sizes of printing characters, and therefore, it has been required to effect sheet feed with a sheet feed pitch always set to be constant and with extremely high accuracy.
FIG. 5 shows a conventional sheet feed mechanism. A metal center shaft 2 extends through a center of a cylindrical sheet feed roller 1 formed of resilient material such as rubber, and a stepping motor not shown is connected through a gear transmission mechanism to one end of the center shaft 2. When the stepping motor is driven, the sheet feed roller 1 is rotated and driven a predetermined amount. A small roller 3 at upstream pressed against the outer peripheral surface of the sheet feed roller 1 and a small roller 4 at downstream are rotatably arranged below the sheet feed roller 1.
In the aforementioned conventional sheet feed mechanism, a sheet 5 is inserted between the lower side of the sheet feed roller 1 and the small rollers 3 and 4, and the stepping motor is driven according to a predetermined drive signal whereby the sheet feed roller 1 is rotated at a predetermined angle to feed the sheet 5 held between the sheet feed roller 1 and the small rollers 3 and 4 by a predetermined sheet feed pitch in a printing direction.
However, in the aforesaid conventional sheet feed mechanism, where the sheet 5 is conveyed by the sheet feed roller 1 and the small rollers 3 and 4, at a moment when the rear end of the sheet 5 passes between the sheet feed roller 1 and the upstream small roller 3, the small roller 3 exerts a force in a direction of extruding the sheet and the sheet feed roller 1 becomes rotated early through a backlash portion of the gear of the gear transmission mechanism for driving the sheet feed roller 1. Where a position of a sheet when the rear end of sheet 5 passes between the sheet feed roller and the upstream small roller 3 assumes a position at rear of a predetermined sheet feed pitch, that is, in the vicinity of termination of feed operation, even if the gear transmission mechanism is rotated thereafter through the remaining sheet feed pitch portion, the previous rotation for the backlash portion cannot be absorbed so as to return it to an original positional relation, as a consequence of which there poses a problem in that the sheet feed pitch becomes varied. As a result, print lacks and prints are overlapped, resulting in a problem of extremely lowering a quality of printing. This occurs particularly in case where the thickness of the sheet 5 is large.